It's obvious to everyone that post-Saddam Iraq has an array of internal weaknesses. Hatred between Sunnis and Shiites exploded into near civil war in 2006 - 2007, and animosity between those religious factions continues to roil. Relations between Arabs (both Sunnis and Shiites) and Kurds are also tense and could easily lead to armed conflict, especially over the political status of the oil-rich city of Kirkuk. Iraq's political system borders on the dysfunctional, as evidenced by the continuing inability to form a new government months after national elections.

What is less obvious but could prove extremely troubling for the United States is Iraq's extraordinarily weak position in its region. Iraq is incapable of defending itself from the depredations of several more powerful neighbors. Worse, there is no sign that this situation will improve in the foreseeable future.

Several incidents underscore Baghdad's nearly helpless posture. In 2008, Turkey's anger boiled over regarding the ability of the domestic Kurdish insurgent group PKK to use sanctuaries in Iraqi Kurdistan to launch attacks into southeastern Turkey. Ankara then conducted a military offensive to disrupt those enclaves. There was little that either Baghdad or the Kurdish regional government could do in response to that contemptuous violation of Iraq's sovereignty.

Apparently taking a page from Ankara's playbook, Iran launched punitive assaults in May and June of this year against another Kurdish group that is waging an insurgency against the Tehran regime. Not only have Iranian forces entered Iraqi territory and killed rebel fighters there, but Iran has now reportedly established a fort on Iraq's side of the border. That step suggests that those troops plan to stay a while. As in the case of Turkey's earlier incursion, Baghdad could do little except issue a mewling diplomatic protest.

Iraq has trouble with other neighbors as well. Iraqi and U.S. leaders have complained for years that Syria ignores the passage of al-Qaeda's foreign fighters through its territory on their way to Iraq. There are suspicions that Damascus not only ignores such activity, but assists it. Shiite officials in Iraq have also accused Saudi Arabia of funding and otherwise aiding Sunni factions that are hostile to the Iraqi government.

The reality is that Iraq is a weak player surrounded by neighbors who do not especially wish it well. Quantitative measures alone underscore the extent of the power disparity. According to the latest edition of the International Institute of Strategic Studies' Military Balance, there are 578,269 personnel in Iraq's security forces. But more than 366,000 are Ministry of Interior personnel, trained and equipped to deal with internal-security problems, not foreign military threats. Iraq's bona fide military consists of a fledgling army of 187,000, a navy of 2,000 and an air force of 3,000.

By contrast, Iran deploys nearly 350,000 active-duty army troops, 125,000 naval personnel, and 18,000 air force personnel. They are backed by 125,000 troops in the elite Revolutionary Guard Corps and 350,000 reservists who could be called up on short notice. Turkey fields some 511,000 active-duty forces, including a 402,000-strong army. Those fighters are backed by nearly 379,000 trained reserves. Syria and even Saudi Arabia are also able to deploy more numerous and substantially stronger military forces than Iraq.

A fight between Iraq and any of its neighbors would not be even remotely an equal contest. None of those countries is likely to launch a blatant, full-scale war - although what Iran might do in response to a U.S. attack on its nuclear facilities remains a disturbing uncertainty. The greater danger is that those neighbors will continue to erode Iraq's territorial integrity and prestige, and will seek to manipulate internal Iraqi rivalries for their own advantage. And whoever heads the Baghdad government will have to tread very carefully to avoid antagonizing any of those prickly states.

None of this should come as a surprise to U.S. policy makers. Iraq was once a serious political and military player in the region. It was also, specifically, the principal strategic counterweight to the ambitions of revolutionary Iran. But that ceased to be the case even before Washington finally decided to overthrow Saddam Hussein's regime. Indeed, the damage to Iraq that U.S. forces inflicted during the first Gulf War rendered Baghdad largely ineffectual as a regional factor.

U.S. policy has created a massive power vacuum where a serious regional geostrategic player used to be. It is predictable that other regional actors will seek to fill that power vacuum; indeed, that have already been taking steps to do so.

Predictably, there are calls, both in Iraq and at home, to have the United States stay on past the 2011 troop withdrawal date to play the role of regional stabilizer. Otherwise, advocates warn, there will be a dangerous rivalry for power involving, at a minimum, Iran, Saudi Arabia and Turkey.

That danger is all too real. But U.S. leaders need to consider the costs of keeping American military forces deployed in Iraq and providing a security shield for that country. Such a mission would be expensive. It could also prove quite dangerous. What exactly would the United States do, for example, if NATO ally Turkey decides that it can no longer tolerate the existence of a de facto independent Iraqi Kurdistan, and moves to occupy that region militarily? What would Washington's response be if Tehran attempts to expand its already considerable influence in Shiite-led Iraq. Or if Saudi Arabia continues to aid anti-government Sunni forces?

Shielding a vulnerable protectorate in a hostile neighborhood is neither cheap nor easy. Nor is it a mission that would end anytime soon. Indeed, it would likely go on for decades. U.S. leaders had better think long and hard before undertaking such a thankless commitment.

Ted Galen Carpenter is vice president for defense and foreign policy studies at the Cato Institute.